This invention relates to electrical signal observing devices which have an optical signal as a probe, and convert the waveform of an electrical signal to be measured into an optical intensity waveform to obtain an observable signal.
Generally, repetitive high-speed electrical signals are measured with a sampling type oscilloscope (whose maximum resolution is 20 to 30 pico-seconds) and single phenomena are measured with a real time oscilloscope (whose maximum resolution is about 300 pico-seconds). However, the signals to be measured have increased in speed, and, consequently there is a demand for measurement of electrical signals with higher resolution.
One way to obtain higher resolution is illustrated by the voltage measuring device of U.S. Pat. No. 4,446,425. In this device, an optical modulator comprising a non-linear optical medium (Pockels cell), polarizer, analyzer and compensator is used to sample an electrical signal with a short pulse light beam. However, using the technique of sampling an electrical signal with a short pulse light beam is not applicable to the measurement of single event phenomena.
On the other hand, European Patent Application (OPI) No. 197196 (the term "OPI" as used herein means an "unexamined published application") discloses an electrically-electron optical oscilloscope in which, instead of a short pulse light beam, a continuous wave light beam (CW light beam) is employed.
The above-described conventional device employs an optical modulator including a non-linear optical medium such as a Pockels cell which is generally expensive, and difficult to handle. Furthermore, this conventional device needs a polarizer, analyzer and compensator in addition to the non-linear optical medium, and therefore its optical system is unavoidably complicated and its adjustment is intricate and troublesome. The optical modulator using the non-linear optical medium merely changes light transmittance using the change in light polarization, and does not have amplification capabilities. Therefore this device cannot effectively utilize the incident light and has a low S/N ratio.